1. Field of the Invention
The present invention relates to an optical recording medium in which recording and reproducing of information are performed by a laser beam irradiation, in particular, a one-side multilayer optical recording medium which is capable of large-capacity, high-density recording, and method and apparatus for optical recording and reproducing thereof.
2. Description of the Related Art
In compact disc (CD) or DVD system in general, recording of binary signals and detection of tracking signal are performed by reflectance modulation arisen from the light interference from the bottom and mirror surface parts of the concave pit. In late years, recordable compact disc (CD-R) having an organic dye film as a recording layer and a phase-change, rewritable compact disc (CD-RW: CD-Rewritable) are increasingly used as CD-compatible (compatible) medium for reproducing, and various proposals are given for DVD system such as recordable DVD and phase-change rewritable DVD. Of the optical recording medium having conventional organic dye thin film as recording layer, the ones containing phthalocyanine compound, cyanine dye, phenalene dye, naphthoquinone dye, and the like in the organic dye thin film are known.
A recordable compact disc-type recording medium (CD-R), in which an organic dye, a metal reflective layer, and a UV resin protective layer are sequentially disposed on a substrate, requires high reflectance to meet the CD standards. Therefore, it was required to develop a highly stable organic dye with high refraction index at a reproduction wavelength (770 nm to 830 nm) (reproduction wavelength of DVD±R, which is recordable DVD, is 630 nm to 680 nm).
With respect to CD-R and DVD±R, a number of optical recording medium have been proposed in which cyanine dye, phthalocyanine dye or azo metal-chelate dye, or the like are used as recording material paring with metal reflective layers.
Guiding grooves for signal recording, which is called “groove land” and consist of irregularity forms, are formed in an optical disc. The side caving in a form of concavity in a direction getting farther from the site from which a laser beam is irradiated for recording and reproducing is called “land” and the side bulging in a form of convexity in a direction approaching the site is called “groove”. The recordable capacity is decided by the pitch (track pitch) of guiding grooves which are recordable and reproducible. For example, DVD+R or DVD+RW employs groove recording method by which information is recorded only in grooves and track pitch is 0.74 μm when recording capacity is 4.7 GB. Furthermore, land groove recording method in which recording takes place in both groove and land is also known and is employed in DVD-RAM.
Recently, to increase the storage capacity of an optical disc, a multiple data layer system has been proposed. An optical disc having two or more data layers is accessible to various layers by changing its lens focal point.
A one-side multilayer, recordable disc using organic dye layer is described in International Publication No. WO00/016320 and International Publication No. WO00/023990, however, information layer is a repeated single organic dye layer and it is not a layer composition considering DVD, etc. compatibility.
A one-side multilayer, recordable disc is described in Japanese Patent Application Laid-Open (JP-A) No. 2001-084643 and JP-A No. 2001-101709, however, inorganic material is used in the recording layer, posing a cost problem.
Further, groove shapes and push-pull values are not mentioned at all in these known literatures.
A one-side multilayer, recordable optical disc is also described in JP-A No. 2000-082238 and JP-A No. 2000-311384, however, consideration are not given over issues such as optimizing optical properties of each layer and performing appropriate recording and reproducing by stable tracking, etc. and there is no mention about push-pull.
On the other hand, phase-change rewritable CD and DVD dictate the recording information signal using reflectance and phase differences generated by refraction index difference between noncrystal and crystal condition. A phase-change optical recording medium in general has a composition consisting of lower protective layer, phase-change recording layer, upper protective layer and reflective layer on a substrate (These are collectively called “information layer”) and multiple interactions of these information layers can be used to control reflectance and phase differences to become CD or DVD compatible. Within a range of reflectance being decreased by about 15% to 25%, CD-RW can be made CD compatible in terms of recording signal and groove signal and reproducing is possible for a CD drive with added amplification system to cover low reflectance.
In addition, because phase-change optical recording medium can perform erasing and re-recording process only by power modulation of one conversing light beam, recording includes overwrite recording in which recording and erasing are performed simultaneously in the phase-change optical recording medium such as CD-RW or rewritable DVD (DVD+RW, DVD−RW, etc.). Crystal, noncrystal or combination of these conditions can be used for phase-change recording of information and it is possible to use multiple crystal phases, however, rewritable phase-change optical recording medium that have been put in use currently recognize unrecorded and erased condition as crystal condition and perform a recording by forming amorphous marks.
Chalcogen element, that is, chalcogenide alloys including S, Se and Te are mainly used for material of phase-change recording layers. Examples include GeSbTe series mainly consist of GeTe—Sb2Te3 pseudo-binary alloy, InSbTe series mainly consist of InTe—Sb2Te3 pseudo-binary alloy, AgInSbTe series and GeSbTe series mainly consist of Sb0.7Te0.3 eutectic alloy, and the like. In particular, a recording material of Sb—Te eutectic alloy series, one of the materials popularly used for phase-change optical recording medium such as CD-RW, etc., excels in erasing ratio and has high sensitivity compared to the recording material of GeTe—Sb2Te3 pseudo-binary compound series, and it is known as the best from the viewpoint of definite outline of amorphous part of the recording mark.
As for DVD, it is also possible to increase the number of information recording layers besides density increase of the recording surface in order to improve the recording density of the optical disc. For example, DVD, which has been standardized and commercialized in late years, has a double information recording layer and it is an optical disc in which read out of double information recording layer is possible from one side of the disc. Lately, optical discs having double information recording layer have been developed and reported as recording and reproducing-type DVD (Japanese Patent (JP-B) No. 2702905 and JP-A No. 2002-184032, for example). However, these have not been put in practical use as commercial products.
An information layer disposed on the front side of one-side multilayer optical disc (it is called “the first information layer”) is required to transmit approximately 50% of the laser beam for recording and reproducing of the information layer disposed on the back side (it is called “the second information layer”). Further, the first information layer is required to absorb approximately 40% of the laser beam in order to perform recording with good sensitivity. For above reasons, according to the standards, for example, reflectance of a recordable DVD is 45% to 85% for DVD+R and reflectance of a rewritable DVD is 30% or less for DVD+RW, however, reflectance of the first information layer of a one-side multilayer optical disc further decreases, possibly unstablizing the tracking. On the other hand, because a signal reflected by the second information layer is absorbed by the first information layer, the fraction reaching the light receiving element becomes approximately 50%, therefore, reflectance is thought to be deteriorated.
There is no mention about groove shapes in the Japanese Patent (JP-B) No. 2702905 and though guiding groove width is specified in the JP-A No. 2002-184032, groove depth or push-pull value are not mentioned.